The general goal of these investigations is to elucidate the nature of the mechanisms mediating energy-linked transport of potassium ions across the membranes of rat liver mitochondria. A working hypothesis is suggested that is consistent with currently available data. It is proposed that a non-electrogenic mechanism effects a coupled influx of potassium and hydroxyl ions (or a coupled exchange of potassium ions and protons). It is suggested that the influx of potassium ions measured by means of the isotype K42 is particularly amenable to an analysis of the role of counterion fluxes in the mechanism of cation transfer. Thus, in testing the proposed model, experiments would focus on the pH dependence of the kinetics of energy-linked K42 flux. Effects of chemically specific reagents (e.g. sulfhydryl reagents) on rates of potassium transfer would also be examined. Finally, some experiments would attempt to determine to what extent other cations may be transported by mechanisms similar to or the same as that mediating energized potassium transport.